A couple weeks ago Amazon made some improvements to SimpleDB, with the biggest one being their support of ordering query results. With this update you can now do queries such as getting the newest entries, or get the top rated items. Previously you could code around the lack of ordering on queries through different methods, such as pulling all the results and sorting them in the code, or keeping a list of the 10 most recent items in the DB that is update any time an item is updated. Those methods mean more work for the developer, and if not done carefully much slower response time for end users.
Now with the query ordering SimpleDB is much more attractive for building on top of for many of the typical website applications.
If SimpleDB even with ordering still isn’t enough for you, you need a “real” database or just want to be able to use all the tools built for the existing databases then Elastic Block Store should interest you. It basically functions as a SAN within the Amazon cloud that you can connect to your EC2 instances. As it’s persistent across instances you can use it for permanent storage for items like the files for a relational database. It also has some nice snapshot/backup features putting the data into S3.
If you’re building a site that needs to be able to scale, Amazon is now an even more viable option then it was previously.